1. Field of the Invention
The present invention relates to crystallized glass substrates for data recording media. Particularly, the present invention relates to substrates made of a glass ceramic having high strength and high stiffness useful as various electric and electronic components such as magnetic disk substrates or ferrules.
More specifically, crystallized glasses of the present invention can be easily molded and contain crystal species precipitated by glass heat treatment, such as xcex1-quartz solid solution or quartz-based crystals, enstatite, etc. The present invention also relates to high-Young""s modulus crystallized glass substrates made of a glass composition that can be easily polished and has high Young""s modulus and high surface smoothness as well as an expansion coefficient adaptable to that of other stainless components incorporated into HDDs, said composition being obtained by heat-treating an MgOxe2x80x94Al2O3xe2x80x94SiO2 glass at an appropriate temperature using TiO2 as a nucleating agent to crystallize it.
2. Description of the Related Art
Main components of magnetic storage apparatus such as computers are magnetic recording media and magnetic recording and reproducing heads. Known magnetic recording media include flexible disks and hard disks. Aluminum alloys have been mainly used as substrate materials for hard disks. Recently, the flying height of magnetic heads has been remarkably reduced as the magnetic recording density of hard disk drives for personal computers or servers becomes higher. Thus, there is a demand for extremely high precision in surface smoothness of magnetic disk substrates. However, it is difficult to produce a flat surface from aluminum alloys to satisfy a certain precision level, because aluminum alloys have too low hardness to avoid plastic deformation even if they are polished with high-precision abrasives and machining tools. As the recording density in hard disk drives becomes higher, a demand for reducing deflection or vibration of substrates for magnetic disks during high-speed rotation has also been increasing. However, it is difficult for aluminum alloys to keep some deflection or vibration level required by the specifications of hard disk drives spinning at high speed on the order of 10000 rpm because its Young""s modulus is low. Thus, glass substrates for magnetic disks with high stiffness and high surface smoothness appeared. Among those, chemically strengthened glass substrates and crystallized substrates are well known in which the substrate surface has been strengthened by ion exchange and subjected to a crystallization treatment, respectively.
For example, a glass substrate strengthened by ion exchange is disclosed in JP-A No. 239036/89, which relates to a glass substrate for magnetic disks strengthened by forming a compression stress layer by alkali ion exchange on the surface of the glass substrate comprising, expressed in weight percent, 50-65% SiO2, 0.5-14% Al2O3, 10-32% R2O wherein R represents an alkali metal ion, 1-15% ZnO and 1.1-14% B2O3.
For example, a crystallized glass is disclosed in U.S. Pat. No. 5,391,522, which relates to a crystallized glass for magnetic disks comprising, expressed in weight percent, 65-83% SiO2, 8-13% Li2O, 0-7% K2O, 0.5-5.5% MgO, 0-5% ZnO, 0-5% PbO, provided that MgO+ZnO+PbO=0.5-5%, 1-4% P2O5, 0-7% Al2O3, 0-2% As2O3+Sb2O3and containing fine Li2O.2SiO2crystalline grains as major crystals.
U.S. Pat. No. 5,476,821 discloses a crystallized glass for disks comprising, expressed in weight percent, oxide components such as 35-60% SiO2, 20-35% Al2O3, 0-25% MgO, 0-25% ZnO, provided that MgO+ZnO greater than 10%, 0-20% TiO2, 0-10% ZrO2, 0-2% Li2O, 0-8% NiO, provided that TiO2+ZrO2+NiO greater than 5% and containing spinel crystalline grains as major crystals.
U.S. Pat. No. 5,491,116also discloses a crystallized glass. This crystallized glass is a glass-ceramic article exhibiting a modulus of rupture of at least about 15,000psi, a Knoop hardness of greater than about 760 KHN, a Young""s modulus of about 20xc3x97106 psi and a fracture toughness in excess of 1.0 Mpa.m1/2, wherein major crystalline phases comprise enstatite or its solid solution and spinel crystals, the article having a composition at least 92% of which consists essentially, expressed in weight percent, of 35-60% SiO2, 10-30% Al2O3, 12-30% MgO, 0-10% ZnO, 5-20% TiO2and 0-8% NiO. A substrate for magnetic disks comprising said crystallized glass is also disclosed.
However, the recent increase of magnetic recording density in hard disks accelerates the reduction of the flying height of magnetic heads and the increase of the rotation speed of disks, which imposes more strict requirements on Young""s modulus or surface smoothness of substrate materials for disks. Particularly, the recent increase of data recording density in 3.5-inch hard disks for personal computers and servers strictly requires surface smoothness and flatness for substrate materials. In load/unload mode (ramp load mode), extremely flatter media with low bumps and less variation in bump height are required as compared with CCS (contact start/stop mode). Specifically, Rmax=3-10 nm or less and Ra=0.2-2.5 nm or less are needed. The requirements for the stiffness of substrate materials have also become stricter in order to provide disks with a rotation speed of 10000 rpm or more to increase data processing speed. There is also a demand for substrate materials having a high expansion coefficient of 90xc3x9710xe2x88x927/xc2x0 C. or more to suit to the thermal expansion of stainless components incorporated into hard disks. The capacity and rotation speed of hard disks are expected to be further higher in future, which should necessarily generate a great demand for substrate materials for magnetic data media having even higher Young""s modulus, high expansion, excellent surface flatness, shock resistance or other properties.
Therefore, it is evident that chemically strengthened glasses as disclosed in JP-A No. 239036/89 having a Young""s modulus of about 80 GPa will be insufficient for future strict requirements for hard disks. Glasses chemically strengthened by ion exchange were found to have the disadvantage that they contain a large amount of alkaline components, which are precipitated as alkali ions at pinholes in magnetic films or thin sites such as peripheries of magnetic films or the sites from which glass is exposed after long use in a high-temperature and high-humidity environment, resulting in corrosion or deterioration of the magnetic films. Since conventional glasses strengthened by ion exchange contain a large amount of alkali ions for ion exchange, most of them have too low Young""s modulus (100 GPa) and too low stiffness to meet the requirements by 3.5-inch substrates with high recording density and high rotation speed. Some heat treatment is sometimes applied to improve properties such as coercive force of a magnetic layer after the magnetic layer has been provided on a glass substrate during the process for manufacturing a magnetic data medium, but conventional glasses strengthened by ion exchange as described above cannot achieve a high coercive force because they have low heat resistance as evident from their glass transition temperature of at most 500xc2x0 C.
Conventional crystallized glasses as disclosed in U.S. Pat. No. 5,391,522 are a little superior to the chemically strengthened glasses in Young""s modulus and heat resistance. However, they are difficult to combine with other components of hard disks and to incorporate into a precise mechanism design required for high density because of their thermal expansion coefficient of at most about 70xc3x9710xe2x88x927/xc2x0 C. Another problem is that they have low surface smoothness as shown by surface roughness greater than 10 angstroms, which limits reduction of the flying height of magnetic heads to hinder high-density magnetic recording. Moreover, they cannot be applied to 3.5-inch high-end disk substrates or thin disk substrates because their Young""s modulus is at most about 90-100 GPa.
The crystallized glass for magnetic disks disclosed in U.S. Pat. No. 5,476,821 has a high Young""s modulus of about 140 GPa at maximum. However, it has the disadvantage that major crystals consist of spinel crystals so that the melting temperature or liquid phase temperature becomes high and a too wide difference in hardness between the spinel crystals and the matrix glass makes polishing difficult. Such a high-Young""s modulus crystallized glass is not suitable for mass production because it is difficult to economically prepare with profit. Moreover, thermal expansion coefficient of said glass is at most about 70xc3x9710xe2x88x927/xc2x0 C., which is far lower than that of stainless to hinder precise mechanism design required for high density.
The crystallized glass disclosed in U.S. Pat. No. 5,491,116 is also difficult to polish because of the high content of spinel species and is unsuitable for precise mechanism design required for high density because of the thermal expansion coefficient of at most about 70xc3x9710xe2x88x927/xc2x0 C., which is far lower than that of stainless.
Thus, an object of the present invention is to provide a glass substrate capable of satisfying the requirements for substrates for future magnetic recording media such as small thickness, high stiffness, high heat resistance, high shock resistance, and more specifically to provide a crystallized glass for data recording media such as magnetic disks, which has high Young""s modulus, strength and heat resistance and excellent surface smoothness and surface homogeneity and can be applied to load/unload mode and can be economically prepared.
Another object of the present invention is to provide a process for preparing said crystallized glass.
Still another object of the present invention is to provide a data recording medium such as a magnetic recording medium using a substrate made of said crystallized glass.
The present invention relates to a crystallized glass substrate for data recording media comprising 42-65 mol % SiO2, 11-25 mol % Al2O3, 15-33 mol % MgO and 5.5-13 mol % TiO2 and comprising xcex1-quartz solid solution and enstatite and/or enstatite solid solution as major crystals (hereinafter referred to as crystallized glass substrate (1)).
The present invention also relates to a crystallized glass substrate for data recording media comprising 42-65 mol % SiO2, 11-25 mol % Al2O3, 15-33 mol % MgO and 5.5-13 mol % TiO2 and comprising xcex1-quartz solid solution and enstatite and/or enstatite solid solution wherein the total of the xcex1-quartz solid solution, enstatite and enstatite solid solution is 50% by volume or more in the crystals (hereinafter referred to as crystallized glass substrate (2)).
The present invention also relates to a crystallized glass substrate for data recording media comprising 42-65 mol % SiO2, 11-25 mol % Al2O3, 15-33 mol % MgO and 5.5-13 mol % TiO2 wherein major crystalline phases comprise quartz-based crystals having a diffraction pattern almost comparable to that unique to quartz in the X-ray diffraction pattern and enstatite and/or enstatite solid solution and the specific gravity is 2.9 g/cm3 or more (hereinafter referred to as crystallized glass substrate (3)).
The present invention also relates to a crystallized glass substrate for data recording media comprising 42-65 mol % SiO2, 11-25 mol % Al2O3, 15-33 mol % MgO and 5.5-13 mol % TiO2 wherein crystalline phases comprise quartz-based crystals having a diffraction pattern almost comparable to that unique to quartz in the X-ray diffraction pattern and enstatite and/or enstatite solid solution, the total of the quartz-based crystals, enstatite and enstatite solid solution is 50% by volume or more in the crystals and the specific gravity is 2.9 g/cm3 or more (hereinafter referred to as crystallized glass substrate (4))
In said crystallized glass substrates (1)-(4) , SiO2+Al2O3 may be in the range of 58-80 mol %, MgO/(SiO2+Al2O3) may be in the range of 0.125-0.55, 0-4 mol % ZrO2 may be contained and 0.1-5 mol % Y2O3 may be contained.
The present invention also relates to a crystallized glass substrate for data recording media wherein SiO2+Al2O3 is in the range of 58-80 mol %, the molar ratio MgO/(SiO2+Al2O3) is in the range of 0.125-0.55 and the molar ratio SiO2/Al2O3 is in the range of 2.3-4.2, major crystalline phases comprise quartz-based crystals having a diffraction pattern almost comparable to that unique to quartz in the X-ray diffraction pattern and enstatite and/or enstatite solid solution and the specific gravity is 2.9 g/cm3 or more (hereinafter referred to as crystallized glass substrate (5)).
The present invention also relates to a crystallized glass substrate for data recording media wherein SiO2+Al2O3 is in the range of 58-80 mol % and MgO/(SiO2+Al2O3) is in the range of 0.125-0.55, crystalline phases comprise quartz-based crystals having a diffraction pattern almost comparable to that unique to quartz in the X-ray diffraction pattern and enstatite and/or enstatite solid solution, the total of the quartz-based crystals, enstatite and enstatite solid solution is 50% by volume or more in the crystals and the specific gravity is 2.9 g/cm3 or more (hereinafter referred to as crystallized glass substrate (6)).
Said crystallized glass substrates (5) and (6) may contain 0-4 mol % ZrO2 and 0.1-5 mol % Y2O3.
Said crystallized glass substrates (1)-(6) may have a Young""s modulus of 110 GPa or more, a thermal expansion coefficient of 90xc3x9710 xe2x88x927/xc2x0 C. or more at 30-300 xc2x0 C., and a grain size of the crystalline phases in the range of 10-1000 nm, and they may be free from ZnO.
The present invention also relates to a crystallized glass substrate for data recording media comprising 42-65 mol % SiO2, 11-25 mol % Al2O3, 15-33 mol % MgO, 5.5-13 mol % TiO2 and 0.1-5 mol % Y2O3 and comprising xcex1-quartz solid solution and enstatite and/or enstatite solid solution as major crystals (hereinafter referred to as crystallized glass substrate (7)).
The present invention also relates to a crystallized glass substrate for data recording media comprising 42-65 mol % SiO2, 11-25 mol % Al2O3, 15-33 mol % MgO, 5.5-13 mol % TiO2, 0.1-5 mol % Y2O3 and 0-4 mol % ZrO2 and comprising xcex1-quartz solid solution and enstatite and/or enstatite solid solution as major crystals (hereinafter referred to as crystallized glass substrate (8))
In said crystallized glass substrates (7) and (8), SiO2+Al2O3maybe in the range of 58-80mol %, the molar ratio MgO/(SiO2+Al2O3) may be in the range of 0.125-0.55 and the molar ratio SiO2/Al2O3 may be in the range of 2.3-4.2.
In said crystallized glass substrates (1)-(8) , the data recording media may be magnetic disks.
The present invention also relates to a process for preparing a crystallized glass substrate for data recording media comprising subjecting a glass comprising 42-65 mol % SiO2, 11-25 mol % Al2O3, 15-33 mol % MgO and 5.5-13 mol % TiO2 with the molar ratio MgO/(SiO2+Al2O3) being in the range of 0.125-0.55 and the molar ratio SiO2/Al2O3 being in the range of 2.3-4.2 to a nucleation heat treatment step and a crystallization treatment step to give a crystallized glass wherein the heat treatment temperature in said nucleation heat treatment step is in the range of 760-840xc2x0 C., the heat treatment temperature in said crystallization treatment step is in the range of 950-1150xc2x0 C. and the heat treatment temperatures in said nucleation heat treatment step and crystallization treatment step are selected in such a manner that the resulting crystallized glass has a thermal expansion coefficient of 90xc3x9710xe2x88x927/xc2x0 C. or more at 30-300xc2x0 C. Said process can be used to prepare said crystallized glass substrates (1)-(8) according to the present invention.
In said process, the heat treatment temperature in said nucleation heat treatment step may be in the range of 810-830xc2x0 C. and the heat treatment temperature in said crystallization treatment step may be in the range of 1010-1150xc2x0 C.
The present invention also relates to a crystallized glass substrate for data recording media obtained by a process comprising subjecting a glass comprising 42-65 mol % SiO2, 11-25 mol % Al2O3, 15-33 mol % MgO and 5.5-13 mol % TiO2 with the molar ratio MgO/(SiO2+Al2O3) being in the range of 0.125-0.55 and the molar ratio SiO2/Al2O3 being in the range of 2.3-4.2 to a nucleation heat treatment step and a crystallization treatment step to give a crystallized glass wherein the heat treatment temperature in said nucleation heat treatment step is in the range of 760-840xc2x0 C., the heat treatment temperature in said crystallization treatment step is in the range of 950-1150xc2x0 C. and the heat treatment temperatures in said nucleation heat treatment step and crystallization treatment step are selected in such a manner than the resulting crystallized glass has a thermal expansion coefficient of 90xc3x9710xe2x88x927/xc2x0 C. or more at 30-300xc2x0 C. (hereinafter referred to as crystallized glass substrate (9))
In said crystallized glass substrate (9), the heat treatment temperature in said nucleation heat treatment step may be in the range of 810-830xc2x0 C., the heat treatment temperature in said crystallization treatment step may be in the range of 1010-1150xc2x0 C., the molar ratio SiO2/Al2O3 of said glass may be 3.25 or more and the specific gravity of said crystallized glass may be 2.9 g/cm3 or more.
In said crystallized glass substrate (9), the data recording media may be magnetic disks.
The present invention also relates to a data recording medium comprising a crystallized glass substrate (1)-(9) according to the present invention and a recording layer formed on said substrate. Said recording layer may be a magnetic recording layer.
For example, high-Young""s modulus crystallized glasses of the present invention comprise enstatite (or its solid solution) crystals with high stiffness and xcex1-quartz solid solution crystals or quartz-based crystals with high expansion as major crystals.
We carefully studied to provide a crystallized glass material having a Young""s modulus of 110 GPa or more and a thermal expansion coefficient of 90xc3x9710xe2x88x927/xc2x0 C. or more at 30-300xc2x0 C. As a result, we accomplished the present invention on the basis of the finding that microcrystalline grains such as xcex1-quartz solid solution or quartz-based crystals or enstatite can be precipitated by heat-treating a crude MgOxe2x80x94Al2O3xe2x80x94SiO2-based glass containing TiO2 as an essential component at an appropriate temperature and that the resulting crystallized glass has a high Young"" modulus of 110-180 GPa and a high thermal expansion coefficient of 90xc3x9710xe2x88x927/xc2x0 C. or more at 30-300xc2x0 C. and can be easily molded, and that the resulting glass substrate can easily be polished.
xcex1-Quartz solid solution has a relatively high thermal expansion coefficient (at 30-300xc2x0 C.) (xcex1-quartz solid solution: about 150xc3x9710xe2x88x927/xc2x0 C., xcex2-quartz solid solution: about xe2x88x925xc3x9710xe2x88x927/xc2x0 C., enstatite: about 81xc3x9710xe2x88x927/xc2x0 C., spinel: about 88xc3x9710xe2x88x927/xc2x0 C.) Enstatite is thought to provide a high Young""s modulus even if the grain size is small, because it has a chainlike or laminar crystal form into which glass components penetrate. It is thought that a crystallized glass having a high Young""s modulus and a high thermal expansion coefficient (at 30-300xc2x0 C.) can be obtained by precipitating such xcex1-quartz solid solution with high expansion and enstatite and/or its solid solution as major crystalline phases.